Computer systems based upon the Intel x86 family of microprocessors can employ several different microprocessor modes. Each mode has a defined boundary in terms of memory addresses for program code and data that is configured by the firmware of the microprocessor. A number of the microprocessor modes are “protected” modes in the sense that they operate independently from and are generally unaffected by the operating system that functions over of the microprocessor modes. The microprocessor can execute instructions in several protected modes simultaneously without risk of violating the independent operations of each mode.
In Intel microprocessors, the System Management Mode (SMM) is a protected mode that is designed to have complete authority over the microprocessor. For example, the SMM provides full access to certain I/O functions of the BIOS (basic input/output system), and system management code can be used to control hardware and firmware features independently from the operating system and application software. Additionally, the SMM can be used to store information about the system configuration of a frozen or powered-down device because its operation does not depend upon the correct functioning of the higher-level operating system, application code or device drivers.